1. Field of the Invention
This invention relates to a control system for effective stable regenerative braking from high speed by means of a chopper with a resistor inserted in the DC motor circuit.
2. Description of the Prior Art
In the regenerative brake control of a DC series motor with a chopper device, for example, the motor operates as a DC series generator and therefore the motor voltage fails to exceed a source voltage. This raises a problem of a shortage of braking force at high speeds. In order to solve this problem, the well-known regenerative brake control consists in connecting a resistor in series with the DC motor to provide an aid in the form of dynamic breaking. This resistor is operatively inserted at a high speed where the source voltage is exceeded by the motor voltage, so that the motor voltage less the voltage drop across the resistor is prevented from exceeding the source voltage.
By setting the resistance value of the resistor at an appropriate value, it is thus possible to obtain a required braking force at high speeds. The resistor is shorted when, as a result of deceleration of the motor, the motor voltage is reduced below the source voltage.
However, another problem is that, at the time of the resistor shorting, the source voltage (i.e. voltage across the filtering capacitor) increases by a value equivalent to about the voltage drop across the resistor. In other words, when the resistor is shorted, the motor current tends to increase but is discouraged as the current control system reduces a duty cycle of the chopper device. The reduction in the chopper duty cycle causes an increased current component flowing into the power supply side, thereby increasing the source voltage.
To overcome this problem, the breakdown voltage of the devices actually used must be sufficiently high as compared with the above-mentioned increase in the source voltage, resulting in the disadvantage of high cost of the device. This is especially the case with the thyristor, diode or other semiconductor device used for a chopper whose breakdown voltage must be high enough.